Been a while since this thread has been cluttered up, so might as well get back at it.
I also want to know what the heck is going on. Sorry, I haven't gotten back to it, I've been busy the last few days. I'm also working on building a better breadboard rig (I'll post pics when I'm done that, and you'll see why) which will also include soldered 40-pin headers for my logic analyzer pods so I can more easily figure out what the bus is doing at any given time.
Setting up an HX motherboard to play with would rule out for sure whether this is the HX that's doing it, and not just something on my end. At one point, over a decade ago, I saw an EX at one of my local thrift stores. I didn't pick it up, because I already had my HX and didn't see any point - a decision that I'm still kicking myself in the butt over, because it would be useful not just for testing, but because there are actually layout differences (as I was discussing with dJOS in his SmartWatch thread) between the EX and HX, and I've never had the opportunity to do a proper comparison between them.
I finally just couldn't wait any longer, so right after work was over for the day I set up the HX board RetroGaming Roundup sent me, yanked the cards out of my EX, removed the E000-enable jumper, and stacked them up:
Before pulling them out of the EX I set up an old version of PC Anywhere to run out of autoexec.bat, and... to cut to the chase, the HX board fired right up with the stack installed. Here's a shot of the whole mess (in my cramped and messy tinkering corner) running Check-It:
(My client laptop is an oooold Dell running PC Anywhere's ATerm under DosBox.)
The long and short of it is the card seems to work in an HX, so I remain completely mystified why the copy I sent to you didn't work. The only anomaly I've found so far is that for some reason the machine hangs if the mouse driver loads and finds a mouse attached. The serial port card isn't the problem, PC Anywhere is obviously working fine through it. (And the mouse driver hangs if I try it on either port.) DOS seems to be loading high fine, I wonder if it could be some kind of initialization issue caused by cutemouse trying to load itself high into the relatively tiny amount of RAM that's left over with only 64k of available UMBs, if there's some other compatibility issue with ctmouse on an HX instead of an EX... or a thought just occurred to me that maybe ctmouse likes the V-20 in my EX more than the 8088 in the HX board? Sussing that out might need to wait until I improvise some kind of native keyboard for the HX board because, well, as well as this works it still doesn't work
WELL...
And I just checked, the extended Checkit memory test I kicked off just passed:
I'm actually kind of surprised, because I noticed while I was plugging stuff in this board actually has a nasty cracked RAM socket on it.
(I think it may also need some love in the capacitor department, or something, the sound off the headphone jack is full of audio interference.)
I need to figure out if the ctmouse thing could somehow be related to my board or loading high or is... something else unrelated (I have another V-20, maybe I'll try that), and then I'm thinking of doing a couple *serious* bodges on another copy of the board to try out some mods I have in mind for upping the UMBs to 96k instead of 64k on an HX, but... seriously, color me badly stumped what the deal is with your copy.